Electrophotographic process



United States Patent 3,322,538 ELECTROPHOTOGRAPHIC PROCESS Rowland W. Rcdington, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 30, 1962, Ser. No. 241,377

' 4 Claims. (Cl. 961.1)

This invention relates to electrophotography and particularly to a faster, more sensitive method of electrophotography.

In the usual electrophotographic process a photoconductor is exposed to a light image which generates charge carriers in the photoconductor in the presence of an electric field. A charge carrier, e.g. anelectron in the photoconductor is excited to conduction by a light photon thereby establishing an electron-hole pair. The more mobile of these charge carriers transfers through the photoconductor under the influence of the electric field where it contributes to a latent electrostatic charge pattern corresponding to the original light image. This latent charge pattern is developed by applying an electrostatically attracted powder or toner material to the photoconductor. The powder, more attracted to regions of greater charge, establishes a visible'representation of the latent electroe static image which visible representation can be fixed in place using an adhesive material and/or heat. Alterna- 'tively, the toner image may be transferred to another material and similarly fixed in place.

Another manner of forming electrophotographic images includes the transfer of the charge pattern to a heated thermoplastic material. The thermoplastic material, when subjected to the charge pattern, tends to deform and estab lish undulations in the thermoplastic corresponding to the original charge pattern, as set forth and claimed in the copending application of Sterling P. Newberry, Ser.

No. 862,249, entitled Direct Image Transfer to Thermoplastic Tape, filedDec. 28,1959, and assigned to the assignee of'the present invention. The undulations formed in the thermoplastic material are capable of deflecting light in an appropriate projection apparatus where they are capable of reproducing the recorded image. Alternatively, the photoconductor itself may be formed of thermoplastic material, deformable under the influence of the charge pattern, as described and claimed in the copending application of Joseph Gaynor, Ser. No. 79,260, entitled Information Storage on Deformable Medium, filed Dec; 29, 1960, now United States Patent No. 3,291,601,

and assigned to the assignee of'the present invention.

The aforementioned process, of electrophotography is quite useful foriduplicating documents and the like where the speed of the process is not of great concern. However,

the speed of this process as presently known approximates the order-of-magnitude A .S.A. 10 where the designation A.S.A. refers to the American Standards Association scale. Such a speed is not comparable with filmphotography. The speed, of course, represents the sensitivity of this photoconductive process, and the sensitivity in turn can be measured by the charge energy and effectiveness of the electrostatic image. Energy is limited by several factors including the voltage developed across the photoconductor and the number of charges, e.g..the number of electrons generated per photon of light energy falling upon the photoconductor. The number of electrons per photon in conventional electrophotographic processes is generally materially less than one. And although the voltage can be increased somewhat acros the photoconductor by various means, the combination of these factors has limited the speed of a process .to the approximate value given.

It is accordingly an object of the present invention to provide an improved and more sensitive system for elec- 3,322,538 Patented May 30, 1967 trophotography whose sensitivity is such as to render it comparable to commercial photographic film in speed.

Briefly stated in accordance with an illustrated embodi ment of my invention, a photoconductive layer is disposed in contact with a thin layer of dielectric material to form a laminate and the laminate further includes electrodes on either side thereof, at least one of which should be transparent. The photoconductor is exposed to a light image while the electrodes are coupled in a manner for providing means for a flow of current therebetween to establish a charge pattern representative of the image. The dielectric layer is advantageously [formed to have a large capacitance compared to that of the photoconductor so that most of the latent electrostatic image comprising the charge pattern eventually resides upon the dielectric layer. The charge pattern in transferring from the photoco'nductor to the dielectric layer is multiplied so that a charge corresponding to more than one electron per photon results upon the dielectric layer, thus enhancing the charge image.

The dielectric layer is then removed from the photoconductor and the dielectric layer has applied thereto means capable of differential attraction and arrangement in response to the charge pattern. For example, in accordance with an aspect of the present invention, the development may take place by disposingthe dielectric layer in contact with thermoplastic material. The dielectric layer material desirably has the property of a greater softening at a given elevated temperature than the thermoplastic material so that heat may be applied to a laminate of these two while the thermoplastic is freely deformed by heat establishing meaningful deformations in accordance with the charge pattern. I

At the same time heat is applied to the laminate of the dielectric material and thermoplastic material, a field assisting' development voltage is applied across the laminate, generally of the opposite polarity to that which may 'be initially'applie'd between the photoconductor and the dielectric layer. This field assisting voltage greatly increases the, response of the thermoplastic material to the charge pattern'on the dielectric layer resulting in considerable further enhancement of the charge image. 1

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in'the con-- FIG. 2 is a cross-section illustrating a developing step I in accordance withthe present invention,

FIG. 3 is a plan view of a continuous electrophotographic apparatus accomplishing the process in accordance With the present invention,

FIG. 3a is a plan view of a composite tape layer employed in the FIG. 3 apparatus,

FIG. 4 is a schematic representation of a photoconductor layer in accordance with the present invention, and FIGS. 5a, b and c are further schematic illustrations of dielectric layers in accordance with the present lIlVGH-e tion.

Referring to FIG. 1, a photoconductor 1 is disposed adjacent and in contact with a materially thinner storage layer of dielectric material 2 to form a laminate. The

capacitance of layer 2 is materially greater than that of 3 and 4 on either side of the laminate making ohmic contact with their adjacent layers. At least one of these electrodes, for example electrode 3, is desirably thin and transparent so that light rays from an illuminated object may be imaged upon the photoconductor 1 by means of lens 6.

' A wide variety of materials may be employed in accordance with the present invention, for example, the photoconductor 1 may be formed of cadmium sulfide, cadmium selenide, doped germanium, or lead oxide. The photoconductor layer which is rather thin, e.g. ten microns, desirably has a somewhat lower resistivity than is common in electrophotography. The thinner, e.g. 2 micron thick, dielectric material desirably softens at an elevated temperature to a greater degree than the thermoplastic developing material as hereinafter described. That is, the dielectric layer 2 in general has a lower softening temperature. Wax or low temperature thermoplastic material is suitable. The thin metal electrodes may be copper, chromium or tin, or other suitable contact materials for the photoconductor used.

The photoconductor can be operated in either a charging or discharging mode for causing a transfer of charge in response to a light image. The discharging mode is more adaptable to explanation and will therefore be principally described, it being understood the charging mode, more easily envisioned in other instances, in mathematically and physically identical.

A series connection of a voltage source 7 and a switch 8 is disposed across the electrodes 3 and 4 and is operated alternately with shunting switch 9. The voltage from source 7 is on the order of 700 to 1000 volts D.C. According to the discharging mode, the laminate 1-2 may at first be illuminated by a lighting means (not shown) disposed at object plane 5, with switch 8 in a closed position. The light is arranged to uniformly illuminate the portion of the photoconductor which is to be used. The photons falling upon the photoconductor cause the formation of positive and negative charge carriers, e.g. holes and electrons. One type of carrier, e.g. an electron, drawn by the voltage across the laminate, transfers through the photoconductor to interface 10 between the photoconductor and the dielectric layer, establishing a uniform charge. Application of light at image plane 5 hastens the charging. After charging, switch 8 is opened.

Now, the laminate is exposed to an illuminated object 5 and switch 9, shunting the laminate, is closed for a short period, comparable to the photoconductors lightcurrent time constant. The lighter portions of the object form an image on the photoconductor which tends to reduce the resistance of the photoconductor in that illuminated area, establishing a charge reducing current flow through switch 9. However the charge tends to remain in place in the less illuminated areas, to the extent of illumination absence, and there is therefore built up a differing charge pattern varying point-to-point across the laminate in accordance with the light in the image. The laminate including the electrodes 3 and 4 may be considered during this period as comprising a photoconductor layer in parallel with a capacitor formed of dielectric layer 2.

As indicated the thickness of dielectric layer 2 is advantageously chosen to be small in respect to photoconductor 1. For example, layer 2 may be two microns in thickness while the photoconductor is 10 microns in thickness. Because of this and in addition because of the dielectric nature of the material, the capacitance thereof is higher and most of the signal charge in the combined capacitance resides as electrical stress in the dielectric layer, varying point-by-point in accordance with the image. In addition it has been found that greater charge variation after exposure resides across the dielectric material than the charge variation which would be predicted across the photoconductor alone. That is to say, the charge image is some what amplified.

The charge established across the photoconductor may be viewed as transferring to the dielectric to establish a corresponding charge on the dielectric. The charge pattern having thus transferred, a new charge may be established across the photoconductor and again a portion of this can be viewed as transferring to the dielectric. By a succession of transfers a larger total charge differential is built up than would have been the case if this additional capacitor layer were absent. The charge on the dielectric supplies potential for continuing the transfer process. The reverse action takes place during discharge whereby charge on the dielectric aids in sustaining an enhanced charge flow, varying point-by-point according to the image, so that an enhanced charge image is established on the dielectric. Of course the foregoing charge transfer process is not accurately described as occurring sequentially, but this initial explanation is given in order to help envision the gain in charge achieved by the combination of photoconductor and dielectric layer. The action will be hereinafter more fully analyzed. It will become apparent that a larger number of electrons per photon may be carried by the dielectric layer and therefore a stronger image is formed than would have been possible with the photoconductor alone, in accordance with the ratio C -l-C /C were C is the capacitance of the photoconductor, and C is the capacitance of the dielectric,

Referring to FIG. 2, the dielectric layer 2 together with its electrode 4 is next separated from photoconductor 1 and transported together with electrode 4 to a position in contact with a thermoplastic layer 11 for receiving and permanently recording the electrostatic charge pattern carried by dielectric layer 2. The thermoplastic material, comparable in thickness to the photoconductor, may be a blend of polystyrene, m-terephenyl and a copolymer of weight percent of but-adiene and 5 weight percent of styrene as set forth in the copending application of William E. Glenn, Jr., Ser. No. 8,842, filed Feb. 15, 1960 now United States Patent No. 3,113,179 issused December 3, 1963, said application being a continuation-in-part of application Ser. No. 698,167, filed Nov. 17, 1957 (now abandoned) and of application Ser. No. 783,584, filed Dec. 29, 1958 (now abandoned), all assigned to the assignee of the present invention. The dielectric layer 2 should in some instances exhibit greater softening than thermoplastic layer 11 at the temperatures to which the thermoplastic layer is to be raised, Layer 2 may be formed of wax or may be substantially similar in composition to thermoplastic layer 11, but should preferably be compounded to have greater fluidity, as for example a compound having lower molecular weight. As hereinafter set forth, the layer 2 material may in some instances be of the same composition as layer 11. As also set forth in the aforementioned patent of .William E. Glenn, In, this thermoplastic may be provided with a support layer of optical grade polyethylene terephthalate which may be disposed on the opposite side of an electrode backing 12. The dielectric layer 2 and the thermoplastic layer together form a second laminate including the electrode 4 on the dielectric side of the laminate and electrode 12 which is thin and transparent. The electrode 12 may be formed of chromium metal.

To develop the electrostatic image the laminate is heated (by means not shown) to a temperature sufficient to soften thermoplastic material 11. As indicated, dielectric layer 2 also softens at this temperature. It has been found the thermoplastic layer will deform in accordance with the charge pattern placed thereon at the dielectric-thermoplastic interface 15 producing thickness deformations in the thermoplastic corresponding to the charge pattern. In the FIG. 2 embodiment, negative charge at a point on the surface 15 will he drawn towards a positive electrode 12 and will at the same time physically distort the thermoplastic at such point resulting in an effectively compressed thermoplastic layer at the same point.

At the same time that heating is applied, an electric field is also applied between plates 4 and 12 by means of a D0. voltage source 13 whose connection is completed with switch 14. This voltage source may be of opposite polarity to voltage source 7 in FIG, 1 and of comparable voltage. The deformation process is conderably aided by the application of voltage source 13, because voltage source 13 provides a field which increases the force on the charges in the latent electrostatic image. The application ofthis developing voltage has the effect of considerably enhancing the deformation compared with that which would be present without the application thereof. A field caused to exist between plates 4 and 12 on account of voltage source 13 is large compared to the field which would influence the thermoplastic layer 11 because of the picture charge only. The -deformation sensitivity of the thermoplastic material is made higher by the ratio of these fields. Before cooling, the thermoplastic material 11 may be separated from the dielectric layer 2, and separated from electrode 12 if electrode 12 is not sufficiently transparent. However, if the dielectric layer 2 is chosen to be a transparent material of adequate optical properties, it may be left in place. In such instance it may be formed of material having substantially the same composition as thermoplastic 11 but preferably a different index of refraction.

The thermoplastic layer 11, when it cools, permanently records the electrostatically derived deformations therein and may be employed for the deflection of light in an appropriate projection apparatus, for example, as set forth in the aforementioned patent of William E. Glenn, Jr., or in William E. Glenn, Jr., Patent Re. 25,169 assigned to the assignee of the present invention. Briefly, the thermoplastic material, including its backing, is placed in a projecting device called a Schlieren projection system for transmitting light through the thermoplastic layer. An optical system including certain illuminated slits, and corresponding bars used to block light from the slits, is arranged to block all transmitted light, in the absence of deformation in the intervening thermoplastic material. However if a deformation is recorded in the thermoplastic layer, light is deflected by the deformations around the bars of the optical system and thereby reaches the projection screen, where the image is reproduced.

In recording optical information in this manner. it is frequently desirable to dissect the light image and record it as a series of small elements rather than in a continuous fashion. That is, the image is dissected and recorded in a manner analogous to that in which a television picture is produced by breaking the image into small segments. The charge pattern representing the light image is therefore fragmented into a large plurality of charge bearing areas which cause small separate thickness deformations in the thermoplastic material. Each small area becomes a picture element in the thermosplastic recorded image capable of deflecting light to produce a light response in a projection system. If the image is not thus dissected, a

high intensity area of the recording merely records as a large shallow groove or depression having an edge irregularity capable of deflecting light. By dissecting the image into a large number of elements, the true nature of the recorded information may be projected on a screen in full rather than simply the outline thereof. Such dissection can be accomplished in various ways as described in the aforementioned application of Sterling P. Newberry, Ser. No. 862,249. As a particular example, the image to be recorded on the thermoplastic may be first presented to the photoconductor of FIG. 1 through a screen or reticle for breaking up the image into a plu-- rality of segments. Alternatively, a reticle or fine line pattern as illustrated in FIG. 1a may be first photographed by the photoconductor prior to the exposure to the desired image.

Although permanently recording the latent electrostatic image as thickness of deformations in thermoplastic material is preferred, it is possible to develop the image emarrangement in response to the electrostatic charge pattern, for example a layer comprising toner or electrostatically attractable powder material suspended in a medium, or a layer of material exhibiting chemical change, e.g. oxidation, under the influence of electric charge.

Continuous apparatus for producing electrophotographic images in accordance with the presentinvention is illustrated in FIG. 3. In FIG. 3 a continuous Web or tape of electrophotographic laminate is stored on reel 16. This laminate comprises a flexible photoconductive layer 17 adjacent a dielectric layer 18 formed of a flexible, low softening temperature thermoplastic material, ashereinbefore indicated. Transparent conducting layer 19, which may be formed as a very thin transparent layer of copper, chromium, or tin, provides an electrode on the photoconductor side of the laminate and is grounded by roller 60. A second conducting layer 20 is disposed in contact with the side of the dielectric opposite the photoconductor. This conducting layer has discontinuities as at 21, 22 and 23 separating several individual frames, as frames 24 and 25, for exposure. The layers of the tape laminate are pressed securely together as contained on reel 16 so as to be tightly adherent to one another. Several of the layers are made separable from adjacent layers as by subsequent stripping apart. For example, layers 17 and 18 are made separably adherent. Adhesive material of selectably graded adherency may be employed if desired.

The discontinuous conducting layer 20 has deposited thereon a continuous plastic backing layer 33'. This plastic backing layer 33, which may be for-med of Mylar, is narrower thanthe tape width leaving exposed at least a narrow edge 34 of conducting layer 20 for permitting electrical contact with layer 20. This arrangement is shown in plan View detail in FIG. 3a. Guide rollers desirably do not contact conducting layer 20 unless otherwise indicated.

The photoconductor 17 and its adherent electrode 19 are wound toward a reel 27 driven by a motor 28. Intermediate the reels 16 and 17, an individual frame, for example, frame 24, is exposed to a lamp 29 while at the same time a voltage source, similar to voltage source 7 in FIG. 1, is connected to conducting layer 20 via conducting edge roller 30- and terminals 31 and 32. An electric field therefore exists across the photoconductor which aids a migration of charge carriers, generated by photons from lamp 29, through the photoconductor. The illumination and the application of voltage are discontinued after a charging period.

The remainder of the apparatus, subsequent to lamp 29 along the web, is preferably housed in darkened chambers 35 where exposure of a particular frame to the desired information takes place. Along the tape laminate between tape 29 and reel 27 is disposed an exposure station 36 including a, darkened hood 37 to permit exposure of only a desired frame, and a lens 38 for imaging an object 39 upon laminate frame 25. A shutter means 40 limits the time of exposure. At the same time the laminate is exposed to an object 39 illuminated by lamps 41 and 42, a switch 43 is closed for shunting conducting layer 20 to ground via edge roller 44. The laminate is exposed for a short period for optimally discharging the photocon- 'ductive layer at portions thereof determined according to the light image. As will be appreciated by those skilled in the art, exposure is discontinued while the difference between the light current and dark current discharge flow is substantial.

After exposure the laminate is carried between guide rollers 45 and 46 and a portion of the laminate comprising dielectric 18 as Well as conducting layer 20' is stripped from the laminate as it passes around guide roller 45. The dielectric portion of the laminate is drawn through guide rollers 47, 48 and 49, and upon a takeup reel 50, driven by motor 51. Guide roller 48 is located on the dielectric side of the laminate and protrudes toward a space between guide rollers 47 and 49, located on the opposite side of the laminate. Guide roller 48 acts to press a thermoplastic layer 52, having a transparent metal backing layer 53, onto the dielectric 18 for close contact therewith over the space of one frame for development of the image. The thermoplastic layer, which may be the tape material described in the aforementioned Glenn Patent No. 3,113,179, is drawn from a reel 54, between roller 48 and dielectric layer 18, and upon takeup reel 55 driven by motor 56. Guide roller 48 is grounded to make contact with the metal backing 53 on the thermoplastic material. Guide roller 48 is heated preferably by electrical means (not shown) to cause softening of the thermoplastic 52 as well as the dielectric material 18, while a charge pattern representing an exposed frame is in juxtaposition with the thermoplastic tape. The charge pattern thus presented to the thermoplastic tape by the dielectric layer acts to deform the tape into intelligence bearing undulations as described in regard to the previous embodiment. At the same time the tapes are heated, a voltage source 13 in FIG. 2, connected to conducting backing 20 disposed along this frame, provides a field assisting development voltage between layer 20 and guide roller 48. Application of this voltage greatly enhances the develop ment sensitivity and therefore the extent of deformation in the thermoplastic material 18. Connection to backing layer 20 is provided by edge roller 57 and contacts 58 and 59, the latter being connected to a voltage source, for example, voltage source 13 in FIG. 2 similarly connected. The charge pattern is transformed in this manner into a deformation pattern recorded in the thermoplastic material wound up on reel 55 and which is capable of reproducing a light image in a Schlieren projection system in the manner hereinbefore described.

As will be appreciated by those skilled in the art, the operation of motors 28, 51 and 56 is synchronized so the various webs or tapes do not slip relative to one another. A synchronized arrangement similar to that described in the aforementioned application of Sterling P. Newberry may be utilized.

The greatly increased sensitivity achieved in accordance with the present invention results in photographic speeds greater than 10 times that heretofore attained by the conventional process. For example, with panchromatic response employing the usual photoconductor materials, a speed of A.S.A. 160 can be obtained. In accordance with the present invention these desirable results are attained by transferring a large portion of charge, representative of more than one electron per photon, onto a relatively high capacity dielectric layer. This charge is then effectively transferred to a development material capable of differential attraction and arrangement in response to a charge pattern, using a development voltage field whose effect on the charge pattern is to increase the sensitivity of the development process in proportion to the ratio of fields resulting from the development voltage and the charge pattern, respectively.

The theoretical explanation for a portion of the sensitivity increase will be considered with reference to FIGS. 4 and 5. In electrophotography processes, a photoconductor is used with transient voltages applied thereacross where the resistance of the photoconductor helps determine the time constant of the transient. A cross-section of such a photoconductor layer is indicated schematically in FIG. 4.

Initially, the layer is given a charge Q which decays slowly in the dark and more rapidly in the light. Suppose a charge q of electrons and of holes has been liberated by the light. If both the electrons and holes are mobile, they will be drawn to the electrodes, terminating the photoconduction process. The capitance of the photoconductor will be discharged by just the photogenerated charge, q. In order to have the possibility of a larger effect than this, suppose that one of the carriers is trapped. or much less mobile than the other. The mobile carriers will be drawn to the appropriate electrode, reducing its charge, but the charge on the other electrode will not be changed, and thus the field there will not be changed.

Since this electrode at which the field does not change is the one which would have to supply additional mobile carriers, again the photoconduction process terminates with the arrival of the mobile carriers at the electrode. In this case the signal is less than the photogenerated charge divided by the capacitance of the photoconductor. However when a capacitor is connected in parallel with the photoconductor, it helps maintain the potential across the photoconductor, and the field at the electrode which would supply more carriers is increased, and more charge is transported through the photoconductor.

Three cases are illustrated in FIGS. 5a, b and c. For each case, a cross-section of the layer with the appropriate charges is shown. The fields have been assumed constant for simplicity. In FIG. 5a, both carriers are mobile and in b and 0 only the electrons are mobile. In FIG. 50, an external capacitor, C has been added across the layer. In this case the field at the cathode electrode is increased so that more charge can be transported through the layer.

The addition of the capacitor permits more charge to be transported through the photoconductor and so it is of interest to calculate the maximum voltage signal for this case. The decay of the voltage across the photoconductor is described by for a space limited dark current trap free insulator. In this expression,

A equals the photoconductor area, and L equals photoconductor thickness, 6 equals dielectric permitivity, and ,u equals mobility of carriers not trapped.

Then,

where t=time, V =initial voltage, and f C/OLV For hyperbolic decay equals the dark current divided by C;

equals the photocurrent divided by C and '1' equals the lifetime of trapped carirers. Also where n equals the number of photogenerated carriers, e equals the charge on an electron and ,1. equals the mobility of carriers not trapped.

V is found to be approximately equal to flf aq a L Ap. Ae f In the case of FIG. c, let it be assumed that one electron per photon is generated, in the absence of external capacitor, C That is,

1 electron/ photon In actual practice the first term in the fraction is found to be slightly less than'l electron .per photon. Four- '25 tenths or one-half electron per photon is more realistic.

However it is seen that the total charge generated in response to a photographic image, when using the external capacitor or dielectric layer is proportional to the capacitance ratio C -l-C /C The minimum thickness of the dielectric layer should be larger than the desired deformation amplitude in the thermoplastic material. This is about 1 micron, peak-topeak. Thus a dielectric layer 2 microns thick is about the minimum that could be used where it is desirable to use minimum thickness to increase the term C above.

As also appears from the above ratio, C +C /C the attainable gain can be made larger by making a photoconductor capacitance small. This implies a thick photoconductor, at least thick in comparison to the thickness of the photoconductor layer. However the thickness of the photoconductor layer is in general limited by several factors to a certain range, if we are to really achieve the number of electrons per photons we would calculate with the ratio C +C '/C Stating it another way, the amplification available using a dielectric layer is limited by the ultimate availability of electrons. The ultimate availability of electrons decreases as thickness is increased. Such availability can be increased by using a higher voltage when exposing the photoconductor, as by making the voltage from voltage source 7 in FIG. 1 larger. But a substantially higher voltage tends to increase the dark current excessively. Moreover, excessively high voltages can cause breakdown of the photoconductor. The thickness is limited to values at which the more fundamental parameters of the photoconductor (e.g., lifetime, mobility and electric breakdown field strength) coupled with the thickness still permit desired gain to occur. The thickness of the photoconductor is also limited by the resolution required. For lines per mm. of resolution, the thickness of the photoconductor should not be greater than 10 microns. The voltage of source 7 in FIG. 1 is on the order of 700-1000 volts for the thickness of films used.

During development, illustrated in FIG. 2, as large as possible a field assisting voltage should be used. The thermoplastic material described will support a field as high as 4 10 volts per centimeter.

For purposes of an example, assume the following con- I ditions: 0

7 A storage layer-2a thick with a dielectric constant of 2. A photoconductor-10 thick with a dielectric constant of 2. Develop with 4x10 v./cm. external field provided by a development voltage of 300 volts.

Exposure time=$ sec.

1 0 RC time constant of photoconductor and dielectric=0.1

sec. 7 Development time=0.1 sec. Thermoplastic layer-7.5g thick About 0.4 electron/ photon output capability of the photoconductor without the dielectric layer.

Photoconductor gain:

G: 0.4 x Ce =24 electrons/photon I On the charge storage or dielectric layer the charge/ photon is C 10 QG'X 2.4 2 electron/photon The improvement over an isolated layer with ohmic contacts and other parameters the same is C., -5 tunes Fifty volts is minimum voltage desirable for a detectable signal in thermoplastic when no external field is used. Normal operation uses 300 volts. The deformation scales as the square of the voltage so the minimum force on the charge is If a field of 4X10 v./cm. (e.g. 300 volts on 7.5 1. thick film) is used, a signal corresponding to approximately 8 volts would give a detectable signal, corresponding to the 50 volts above. This is a factor of 6 improvement.

The charge-which would have given'8 volts on a 7 /2,u layer is Q=OV 2X8.885 10- 5 7.5x 10 =2 10- coulombs/cm. 1.3 X 10! electrons/cm. With 2 electrons/ photon, 7x10 photons/cm. are require for minimum signal. For a latitude of 30, 2x10 photons/cm. is the chromatic response, this is a speed of A.S.A. 160. As thus appears a quite rapid response is attained.

To review, this sensitivity is brought about by employing a capacitive layer in conjunction with a photoconductor and a charging-voltage, and then applying a developing voltage while developing the thermoplastic layer.

While I have'shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made Withoutdeparting from my'invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A process of electropho-tography comprising the steps of:

(a) disposing a thin dielectric layer in contact with a photoconductor to form a laminate wherein the thin dielectric layer has the higher capacity and further disposing electrodes on the remaining side of said photoconductor and the remaining side of said layer and applying a voltage between said electrodes,

(b) exposing said photoconductor in said laminate to a light image to cause a differential transfer of charge through said photoconductor to establish a latent electrostatic image corresponding to said light image, I

(c) intercoupling said electrodes during said exposure whereby said charge distributes itself between said maximum requirement. With panphotoconductor and said dielectric layer such that said dielectric layer receives a larger portion of charge than said photoconductor,

(d) separating said dielectric layer and said photoconductor,

(e) applying to said dielectric layer further means capable of differential attraction and arrangement in response to the latent electrostatic image formed,

(f) while at the same time applying a field assisting development voltage between said dielectric layer and said further means, whereby to increase the sensitivity of said electrophotographic process.

2. A process of electrophotography comprising the steps (a) disposing a thin dielectric layer in contact with a photoconductor to form a laminate wherein the thin dielectric layer has the higher capacity and further disposing electrodes on either side of said laminate,

(b) applying a voltage between said electrodes,

(c) exposing said photoconductor in said laminate to a light image to cause a transfer of charge in said photoconductor establishing a latent electrostatic image,

(d) connecting together said electrodes during said exposure,

(e) removing said dielectric layer and its electrode from said photoconductor,

(f) disposing said dielectric layer in contact with a layer of thermoplastic material to form a second laminate, and further disposing an electrode on the thermoplastic side of said second laminate, and

(g) heating said thermoplastic material to a softened condition while at the same time applying a field assisting development voltage to the electrodes on either side of said second laminate to cause selective deformation of said thermoplastic material in response to said latent electrostatic image.

3. The process according to claim 2 wherein said dielectric layer has the property of softening to a greater extent than said thermoplastic material in response to heat applied thereto.

4. A process according to claim 2 wherein at least one of the electrodes of the first laminate is transparent.

References Cited UNITED STATES PATENTS OTHER REFERENCES Cross, Deformation Image Processing, 1MB Technical Disclosure Bulletin, vol. 4, No. 7, December 1961.

NORMAN G. TORCHIN, Primary Examiner.

J. TRAVIS BROWN, Examiner.

A. L. LIBERMAN, C. E. VAN HORN,

Assistant Examiners. 

1. A PROCESS OF ELECTROPHOTOGRAPHY COMPRISING THE STEPS OF: (A) DISPOSING A THIN DIELECTRIC LAYER IN CONTACT WITH A PHOTOCONDUCTOR TO FORMA A LAMINATE WHEREIN THE THIN DIELECTRIC LAYER HAS THE HIGHER CAPACITY AND FURTHER DISPOSING ELECTRODES ON THE REMAINING SIDE OF SAID PHOTOCONDUCTOR AND THE REMAINING SIDE OF SAID LAYER AND APPLYING A VOLTAGE BETWEEN SAID ELECTRODES, (B) EXPOSING SAID PHOTOCONDUCTOR IN SAID LAMINATE TO A LIGHT IMAGE TO CAUSE A DIFFERENTIAL TRANSFER OF CHARGE THROUGH SAID PHOTOCONDUCTOR TO ESTABLISH A LATENT ELECTROSTATIC IMAGE CORRESPONDING TO SAID LIGHT IMAGE, (C) INTERCOUPLING SAID ELECTRODES DURING SAID EXPOSURE WHEREBY SAID CHARGE DISTRIBUTES ITSELF BETWEEN SAID PHOTOCONDUCTOR AND SAID DIELECTRIC LAYER SUCH THAT SAID DIELECTRIC LAYER RECEIVES A LARGER PORTION OF CHARGE THAN SAID PHOTOCONDUCTOR, (D) SEPARATING SAID DIELECTRIC LAYER AND SAID PHOTOCONDUCTOR, (E) APPLYING TO SAID DIELECTRIC LAYER FURTHER MEANS CAPABLE OF DIFFERENTIAL ATTRACTION AND ARRANGEMENT IN RESPONSE TO THE LATENT ELECTROSTATIC IMAGE FORMED, (F) WHILE AT THE SAME TIME APPLYING A FIELD ASSISTING DEVELOPMENT VOLTAGE BETWEEN SAID DIELECTRIC LAYER AND SAID FURTHER MEANS, WHEREBY TO INCREASE THE SENSITIVITY OF SAID ELECTROPHOTOGRAPHIC PROCESS. 